The reinforcing phenomenon by blending soft elastomeric materials with rigid colloidal filler particles is, despite of its technological relevance, still poorly understood on the microscopic level although several macroscopic approaches have dealt with some of the mechanisms successfully. In order to contribute to bridge this gap, a study of a composite made up of a commercially silica-based hard phase and of a PI polymer matrix has been performed. The undeformed `reference' state for such studies has been investigated by means of the small angle scattering technique on partially labeled samples. The single chain behaviour was isolated from neutron scattering although the condition of composition matching was not met, whereas the precipitated fractal filler structure was characterized by synchrotron X-ray scattering. The effective 3-component system has been successfully described through a combined approach of neutron and X-ray data and relationships dealing with 4-component structures have been presented. This work deals for the first time directly with the soft matrix in a reinforced 2-phase composite and is the basis for the treatment of anisotropic data in terms of both filler and chain dependence on uniaxial strain. (C) 2003 Elsevier Ltd. All rights reserved.